I have several excel documents containing PowerPivot tables. They all connect to a SSAS Tabular service (on SQL 2012) via a .odc connection file, all of which is hosted on SharePoint 2013, using the PowerPivot Gallery as the default view. It is imperative that the documents refresh when they are openened, because the data is filtered depending on the user that views the data. However, when I enable the 'refresh on open' feature for the data connection, the gallery refuses to show any thumbnails. When I uncheck this feature the thumbnails show as expected. I'm guessing this might have something to do with time-out.
The workbooks in the screenshots are copies of each other, except that the first refreshes on open, and the other doesn't.
PS: if I run GallerySnapshot.exe tool by hand, snapshots are generated without errors for all the workbooks.